I would keep it and use it as well. I have an old Canon S5IS with a few bits n pieces and every so often I get it out and play with it. I have my Nikon that I prefer but dont have the heart to part with my Canon. I have also kept my old point n shoots too.

What collection of Canon lenses do you currently own? How much money have you already invested in the Canon system? What lens did the Nikon come with? If you only have kit lenses for both systems then to me it is a no brainer and sell one and invest in just one system.